Bruce Schneier asks the question. It is true that plenty of them are dumb. But not all of them are. And I’d like to point out a snippet from a paper I wrote a few years back:
Claude Berrebi notes that “[i]f there is a link between income level, education, and participation in terrorist activities, it is either very weak or in the opposite direction” (Berrebi, 42). In a later study, Efraim Benmelech and Berrebi show the importance of human capital in suicide bombers and how suicide terrorism is usually accomplished by educated individuals (Benmelech and Berrebi, 2-3). And finally, Krueger and Maleckova show not only that higher-income Lebanese are more likely to join Hezbollah (Krueger and Maleckova, 135), but also that “there is little evidence to suggest that a deteriorating economy or falling expectations for the economy” resulted in the September 2000 Intifada (128), meaning that economic conditions do not appear to be a significant driver for terrorist activities.
Benmelech, Efraim and Claude Berrebi. “Attack Assignments in Terror Organizations and the Productivity of Suicide Bombers.”
Krueger, Alan B. and Jitka Maleckova. “Education, Poverty and Terrorism: Is there a Causal Connection?”
In other words, although many terrorists are stupid, these organizations do not select for stupidity, and there is some evidence that they value highly-educated members.